In this paper, a microcell is designed for enhancing the uplink capacity of an existing terrestrial cellular code division multiple access (CDMA) system. The proposed designs involve integration of a high altitude platform station (HAPS) and the terrestrial system. By adjusting the projection of a phased array antenna onboard the HAPS, two designs of microcell are developed in the integrated system. One is that main lobe directions of the antenna coincide with the locations of terrestrial base stations; the other instead has a projection pattern with main lobes pointing toward terrestrial cell boundaries. Numerical analysis is performed and the results show that the uplink capacity is significantly enhanced under both our proposed designs, compared with a purely ground-based or a purely HAPS CDMA system; furthermore, the latter design can provide more than 15% higher uplink capacity over the former one.